Two factors affecting the performance of a high voltage insulating material are its tracking resistance and its erosion resistance. Tracking refers to the formation of conducting paths (tracks) of deteriorated material on the surface of the insulating material, caused by surface electric discharges. A tracking failure occurs when a track bridges the gap between two or more conductors, leading to an electrical breakdown. Erosion refers to the progressive wearing away of the insulating material by the electrical discharges, with failure eventually occuring because too much insulating material has been worn away.
Organopolysiloxanes (also called silicones) are common high voltage insulating materials, because of their electrical properties, processability (including moldability), physical properties, chemical inertness and other desirable characteristics. Organopolysiloxanes employed in high voltage equipment typically contain additives for enhancing their tracking and/or erosion resistance (hereinafter referred to as anti-tracking additives).
One well-known anti-tracking additive is alumina hydrate (also referred to as aluminum hydroxide, alumina trihydrate, hydrated alumina or ALTH) alone or in combination with other additives such as metal oxides. Illustrative disclosures relating to alumina hydrate include Elliot, U.S. Pat. No. 3,965,065 (1976); Penneck, U.S. Pat. No. 3,969,308 (1976); Penneck, U.S. Pat. No. 4,001,128 (1977); Cammack, II et al., U.S. Pat. No. 4,100,089 (1978); Penneck et al., U.S. Pat. No. 4,189,392 (1980); Penneck, U.S. Pat. No. 4,399,064 (1983); Clabburn et al., U.S. Pat. No. 4,431,861 (1984); Penneck, U.S. Pat. No. 4,521,549 (1985); Adkins, U.S. Pat. No. 4,822,830; and Kunieda et al., U.S. Pat. No. 5,369,161 (1994). It has also been proposed to use alumina hydrate as an additive for other purposes, such as improving flame retardancy: Bobear, U.S. Pat. No. 4,288,360 (1981). The alumina hydrate is sometimes combined with other additives, such as other metal oxides or phosphorus compounds.
Another class of additives are the platinum compounds, which are normally used for improving flame retardancy: Lauret al., U.S. Pat. No. 3,635,874 (1972); Pfeifer et al., U.S. Pat. No. 3,711,520 (1973); Milbert, U.S. Pat. No. 3,821,140 (1974); Bargain, U.S. Pat. No. 3,839,266 (1974); Hatanaka et al., U.S. Pat. No. 3,862,082 (1975); Itoh et al., U.S. Pat. No. 3,936,476 (1976); Matsushita, U.S. Pat. No. 4,110,300 (1978); Bobear, U.S. Pat. No. 4,288,360 (1981); Ackermann et al., U.S. Pat. No. 4,419,474 (1983); and Derwent WPI Abstract No. 76-82267X/44 (abstract of JP-50-097644 (1975)). Again, other additives may be simultaneously employed, either for flame retardancy also or for other purposes such as heat stabilization or tracking/erosion resistance.
Lastly, many other additives have been added to organopolysiloxanes, for diverse purposes ranging from the aforementioned ones to thermal conductivity, reinforcement, and high temperature stability. These additives include metal oxides, silica, and metal salts. Exemplary disclosures in this vein are: Koda et al., U.S. Pat. No. 3,884,950 (1975); Cole et al., U.S. Pat. No. 4,604,424 (1986); Szaplonczay et al., U.S. Pat. No. 4,897,027 (1990); Wolfer et al., U.S. Pat. No. 5,008,317 (1991); Bosch et al., U.S. Pat. No. 5,023,295 (1991); Mazeika et al., WO 95/06552 (1995); Rowe et al., EP 0,218,461 A2 (1987); and GB 1,538,432 (1979). In summary, an organopolysiloxane used as a high voltage insulating material may contain a complex additive package.
An organopolysiloxane may be formed into a complex shape for a particular end use. For instance, the aforementioned Mazeika WO '552 discloses a high voltage insulator having a shedded organopolysiloxane housing which has been molded by a process leaving no longitudinal mold lines (which are more susceptible to tracking failure). The combination of the critical placement of the mold lines and the complex shape of the molded part places stringent demands on the organopolysiloxane. It must flow readily enough to properly fill the mold cavity and, after molding, be sufficiently compliant to demold. However, many prior art anti-tracking additives interfere with the molding process for one reason or another. Alumina hydrate makes the molded part difficult to demold, especially at the loading levels needed for effectively improving antitracking resistance, about 15 parts per hundred by weight or greater, usually more than 100 phr in commercial embodiments. In our experience, alumina hydrate levels greater than 75 phr make demolding difficult. Many moldable organopolysiloxane compositions are cured (crosslinked) in the mold by via a vinyl-hydride addition reaction: ##STR1## Curing may be effected by a platinum catalyst, such as hexachloroplatinic acid. Amounts of platinum in excess of those used for curing may be used an additive; however, in such instances the platinum additive may also catalyze the curing reaction, leading to premature curing, or scorching. Another disadvantage of platinum as an additive is its high cost.
Further, greater and greater performance demands are placed on high voltage insulating materials, for example where power generating or distribution plants are built in polluted coastal areas--locations where tracking and erosion are particularly severe problems. Consequently, there is a need for more effective anti-tracking packages which do not exhibit the limitations of prior art packages.